Simple:
* As lively as a puppy.
* As energetic as a hummingbird.
* As bouncy as a rubber ball.
More descriptive:
* As energetic as a whirlwind sweeping through a field.
* As full of life as a bubbling spring.
* As tireless as a marathon runner in the final stretch.
Figurative:
* Their energy was like a fire that couldn't be contained.
* They moved with the grace and speed of a dancer in a whirlwind.
The best simile will depend on the specific context and the type of energy you want to convey.